The small subunit of the bacterial ribosome is a complex organelle composed of one RNA molecule and 21 different proteins. It is responsible for recognition of initiation sites on mRNA and specific binding of the correct charged tRNA molecule corresponding to each mRNA codon during protein synthesis. In order to understand the molecular mechanisms by which the small subunit accomplishes its role in initiation protein synthesis and regulation of translational fidelity, it will be necessary to determine the three-dimensional structure of the ribosome. The mapping of ribosomal proteins on the surface of the subunit by electron microscopy of antibody-ribosome complexes has produced a low resolution map of the location of one or more antigenic determinants of many proteins on the surface of the subunit. We shall use antibodies specific for particular regions of these ribosomal proteins to determine the orientation of the proteins in the subunit and in protein deficient particles such as the RI particle. In this way a higher resolution map of the orientations of the individual proteins will be produced. In some cases, small subunit proteins such as S4 do not have exposed antigenic determinants on the surface of the intact subunit. These proteins may, however, be mapped on various subribosomal particles. The proteins responsible for the masking of specific antigenic determinants will be determined by measuring the exposure of antigenic determinants in protein deficient reconstituted particles.